1. Field of the Invention
The invention relates to the addition of an LED (light emitting diode) to an illumination device to be used as reference light source to maintain brightness over lifetime.
2. Description of Related Art
Lamps and displays using LEDs (light emitting diodes) for illumination are becoming increasingly popular in many different markets. LEDs provide a number of advantages over traditional light sources, such as fluorescent lamps, including low power consumption, long lifetime, and no hazardous material, and additional specific advantages for different applications. For instance, LEDs are rapidly replacing Cold Cathode Fluorescent Lamps (CCFL) as LCD backlights due to smaller form factor and wider color gamut. LEDs for general illumination provide the opportunity to adjust the color or white color temperature for different effects. LED billboards are replacing paper billboards to enable multiple advertisements to timeshare a single billboard. Further, projectors that use LEDs as the light source may become popular in mobile handsets, such as smartphones, in the near future. Likewise, Organic LEDs or OLEDs, which use multi-colored LEDs directly to produce light for each display pixel, and which use arrays of organic LEDs constructed on planar substrates, may also become popular for many types of display applications.
Although LEDs have many advantages over conventional light sources, such as incandescent and fluorescent light bulbs, a disadvantage of LEDs is that the brightness produced by a fixed current can change over time. For instance, during the earliest phase of an LED life cycle, the optical output power can increase or decrease depending on whether defects in the active region grow or shrink. During the later phases of an LED's lifecycle, the optical output power for a given drive current continually decreases until replaced. Unlike a conventional incandescent or fluorescent light bulb that typically fails catastrophically, a typical LED lamp will just get dimmer over time, which can be an issue if one lamp in an array of LED lamps has to be replaced before the others. The new lamp typically will appear brighter than the rest, which may not be acceptable in some applications.
Although most commercially available LED lamps today do not compensate for light output degradation over time, some lamps, such the LR6 available from Cree, have photo-detectors and optical feedback circuitry to monitor and adjust output intensity. Such lamps, however, are typically more expensive than those without such compensation circuitry. Additionally, such compensation circuitry can be adversely affected by temperature and other variations in operating conditions, which either degrade performance or require cost and complexity to compensate.
As such, a need exists for a improved techniques to maintain a fixed brightness produced by an LED lamp without the cost and complexity of conventional photo-detector based optical feedback circuitry.